Silicon has been a dominant semiconductor material in the electronics industry, but it does have a disadvantage in that it has poor optical activity due to an indirect band gap. This poor optical activity has all but excluded silicon from the field of optoelectronics. In the past two decades there have been highly motivated attempts to develop a silicon-based light source that would allow one to combined an integrated digital information processing and an optical communications capability into a single silicon-based integrated structure. For a silicon-based light source (Silicon Light Emitting Diode (LED)) to be of any practical use, it should (1) emit at a technologically important wavelength, (2) achieve its functionality under practical conditions (e.g. temperature and pump power), and (3) offer competitive advantage over existing technologies.
One material that has gathered much international attention is erbium (Er) doped silicon (Si). The light emission from Er-doped Si occurs at the technological important 1.5 micron (μm) wavelength. Trivalent erbium in a proper host can have a fluorescence of 1540 nm due to the 4I13/2→4I15/2 intra-4f transition. This 1540 nm fluorescence occurs at the minimum absorption window of the silica-base telecommunication fibre optics field. There is great interest in Er doping of silicon as it holds the promise of silicon based optoelectronics from the marriage of the vast infrastructure and proven information processing capability of silicon integrated circuits with the optoelectronics industry. Theoretical and experimental results also suggest that Er in Si is Auger-excited via carriers, generated either electrically or optically, that are trapped at the Er-related defect sites and then recombine, and that this process can be very efficient due to strong carrier-Er interactions. However, if this strong carrier-Er interaction is attempted in Er-doped bulk Si, the efficiency of the Er3+ luminescence is reduced at practical temperature and pump powers.
Recently, it has been demonstrated that using silicon-rich silicon oxide (SRSO), which consists of Si nanocrystals embedded in a SiO2 (glass) matrix, reduces many of the problems associated with bulk Si and can have efficient room temperature Er3+ luminescence. The Si nanocrystals act as classical sensitizer atoms that absorb incident photons and then transfer the energy to the Er3+ ion, which then fluoresce at the 1.5 micron wavelength with the following significant differences. First, the absorption cross section of the Si nanocrystals is larger than that of the Er3+ ions by more than 3 orders of magnitude. Second, as excitation occurs via Auger-type interaction between carriers in the Si nanocrystals and Er3+ ions, incident photons need not be in resonance with one of the narrow absorption bands of Er3+. However, existing approaches to developing such Si nanocrystals have only been successful at producing concentrations of up to 0.3 atomic percent of the rare earth element, which is not sufficient for practical applications.
In general, manufacture of type IV semiconductor nanocrystals doped with a rare earth element is done by ion implantation of silicon ions into a silicon oxide layer, followed by high temperature annealing to grow the silicon nanocrystals and to reduce the ion implantation damage. The implantation of Si ions is followed by an ion implantation of the rare earth ions into the annealed silicon nanocrystal oxide layer. The resulting layer is again annealed to reduce the ion implant damage and to optically activate the rare-earth ion.
There are several problems with this method: i) it results in a decreased layer surface uniformity clue to the ion implantation; ii) it requires an expensive ion implantation step; iii) it fails to achieve a uniform distribution of group IV semiconductor nanocrystals and rare-earth ions unless many implantation steps are carried out; and iv) it requires a balance between reducing the ion implant damage by thermal annealing while trying to maximise the optically active rare-earth.
To diminish the above drawbacks, Plasma Enhanced Chemical Vapor Deposition (PECVD) has been utilised to make type IV semiconductor nanocrystal layers. The prepared layers are then subjected to a rare-earth ion implantation step and a subsequent annealing cycle to form the IV semiconductor nanocrystals, and to optically activate the rare-earth ions that are doped in the nanocrystal region. Unfortunately, the layers prepared with this method are still subjected to an implantation step, which results in a decrease in surface uniformity.
Another PECVD method that has been used to obtain a doped type IV semiconductor crystal layer consists of co-sputtering together both the group IV semiconductor and rare-earth metal. In this method, the group IV semiconductor and a rare-earth metal are placed into a vacuum chamber and exposed to an Argon ion beam. The argon ion beam sputters off the group IV semiconductor and the rare-earth metal, both of which are deposited onto a silicon wafer. The film formed on the silicon wafer is then annealed to grow the nanocrystals and to optically activate the rare-earth ions. As the rare earth metal is in solid form, the argon ion beam (plasma) is only able to slowly erode the rare earth, which leads to a low concentration of rare earth metal in the deposited film. While higher plasma intensity could be used to more quickly erode the rare earth metal and increase the rare earth concentration in the film, a higher intensity plasma damages the film or the group IV semiconductor before it is deposited. The plasma intensity is therefore kept low to preserve the integrity of the film, therefore limiting the rare earth concentration in the film. The doped group IV semiconductor nanocrystal layers made through this method have the drawbacks that: i) the layer does not have a very uniform distribution of nanocrystals and rare-earth ions, ii) the layer suffers from upconversion efficiency losses due to rare-earth clustering in the film, and iii) the concentration of rare earth metal in the layer is limited by the plasma intensity, which is kept low to avoid damaging the layer.
The concentration of the rare earth element in semiconductor nanocrystal layers is preferably. As high as possible, as the level of photoelectric qualities of the film, such as photoluminescence, is proportional to tire concentration. One problem encountered when a high concentration of rare earth element is present within the semiconductor layer is that when two rare earth metals come into close proximity with one another, a quenching relaxation interaction occurs that reduces the level of photoelectric dopant response observed. The concentration of rare earth element within a semiconductor film is thus balanced to be as high as possible to offer the most fluorescence, but low enough to limit the quenching interactions.